Heretofore conventional processes which use the system for measuring the forward movement of machine tool carriages to measure the workpiece operate with a test probe to generate a signal which, when it is given, determines the position of the machine carriage in relation to a reference point on the machine tool. In DE-OS 31 03 166 a workpiece measuring process for a numerically controlled machine tool is disclosed with which the radius of a workpiece can be measured. For this purpose the machining tool is moved against the outer surface of the workpiece and a tape of the associated control system stores coded, digital information units which represent predetermined positions for the tip of the cutting tool. This information is supplied by a tape reader to a register via a digital computer. A feedback signal is connected to a counter, the content of which represents the instantaneous position of the cutting tip, the counter being set to zero beforehand by a signal generated when the cutting edge abuts against a reference value transmitter located on the axis of the workpiece. The register and the counter are connected to a comparator which has an output which represents the instantaneous difference between the contents of the register and of the counter. The output of the comparator generates a signal from which the radius of the workpiece is determined. The contact between the cutting tip of the tool and the workpiece is detected either by an electrical pressure sensor in the tool holder or by an electric current circuit which is closed when the tool touches the workpiece. Such arrangements are complicated and expensive, and are susceptible to faults. For example calibration of the tool is necessary before each measurement.
According to DE-OS 34 46 138 a measuring device integrated in the tailstock quill is proposed for a further NC-controlled machine tool having a tailstock. While this measuring device does not need to be removed from the working chamber during machining it only offers restricted possibilities and likewise involves considerable expense, since the tailstock does not need a measuring device of its own. The presence and the position of a workpiece in the machine can be determined by this measuring device.
Finally, a tool measuring device is disclosed in DE-OS 37 42 573 which does not require any arrangements for generating signals and uses the stroke measuring system of the machine itself to measure the tool. The signal is obtained by monitoring the contouring error of the feed drive while the tool is moved against a fixed stop. Since cutters are relatively delicate elements various measures have to be taken to prevent the cutter from being damaged on the stop. The tool correction is given by the difference between the target position and the actual position reached.